1. Field of the Invention
The present invention relates to an optical memory through which information can be recorded, reproduced, and erased on the basis of phase transition of the memory caused by irradiating a laser beam or the like, and more specifically to an optical memory which can maintain recorded information under stable condition for a long term.
2. Description of the Prior Art
As optical memories through which information can be recorded, reproduced and erased, magneto-optical mediums and phase transition mediums have been well known.
In the phase transition type optical memories, information is recorded and erased on the basis of reversible phase transition in crystal structure of the memory which is caused by irradiating a laser beam.
For instance, the recording and erasing are effected by the utilization of change in crystal structure between a crystalline state and a non-crystalline state (referred to as amorphous state) or between two crystalline states. That is, when a laser beam is irradiated upon a recording film to quickly heat the film and then quench it, phase transition is effected from an equilibrium phase to a non-equilibrium phase to record information. On the other hand, when the laser beam is irradiated upon the recording film to heat it and then cool it slowly, the non-equilibrium phase is returned to the equilibrium phase to erase the recorded information. Further, the recorded information can be reproduced by irradiating a laser beam upon the film to detect the change in reflectivity and/or transmittance between the non-equilibrium phase area at which information is recorded and the equilibrium phase area at which no information is recorded. Here, the non-equilibrium phase implies a non-crystal structure or a metastable phase crystal structure.
As the above optical memories of phase transition type, conventionally, chalcogenide base semiconductors such as Te, Ge or the like have been used.
In the prior art optical memories of phase transition type, however, since the crystallization temperature of pure Te lies at near room temperatures (about 10.degree. C.), after a laser beam including recording information has been irradiated to change the phase of a pure Te film from the equilibrium phase to the non-equilibrium phase for information recording, there exists a problem in that the non-equilibrium phase state returns to the equilibrium phase state due to a change with the passage of time and thereby the recorded information is erased naturally. On the other hand, where Ge is adopted for the memory, since a Ge film is chemically unstable, the Ge film is readily eroded within the atmosphere, and therefore it is impossible to keep information stably recorded thereon.
As described above, since the prior art optical memories adopt a chalcogenide base semiconductor as the recording thin film, there exists a problem in that it is impossible to maintain a stable recording state for a long time.